The present invention relates to connections between printed circuit boards.
Certain computer systems may employ a serial bus to transmit signals between a memory controller and memory. An example of such a serial bus has been defined by Rambus Corporation of Mountain View, Calif. That bus, often called the Direct Rambus memory channel, enables transmission of high speed, pipelined signals between a memory controller and memory. A memory card or module coupled to the bus may contain a number of high speed DRAMs, which have a Rambus developed architecture. Such memory devices are often called xe2x80x9cRambus DRAMsxe2x80x9d or xe2x80x9cRDRAMs.xe2x80x9d
The Direct Rambus memory channel may require signals to travel from a motherboard to a memory card. The memory card typically is inserted into a socket, which is mounted to the motherboard. That socket may introduce an impedance discontinuity into the signal line that could adversely affect system performance, e.g., by requiring reduction in the maximum frequency at which high speed, pipelined electrical signals may be driven along the channelxe2x80x94to prevent signal reflection that may degrade signal quality.
FIG. 1 represents a Direct Rambus memory channel, which shows memory cards 10 and 20 inserted into sockets 15 and 25, which have been mounted to a motherboard. Memory devices 5 are mounted onto cards 10 and 20. (Dashed boxes 6 serve to indicate that cards 10 and 20 may include memory devices in addition to those shown, which may be mounted to both sides of the cards. For example, as shown here, each memory card may include 16 memory devicesxe2x80x948 mounted to each side.) High speed, pipelined signals travel from memory controller 30 through cards 10 and 20 to termination resistors 35. As those signals travel from one end of the memory channel to the other, sockets 15 and 25 act as inductors. They also apply a small amount of capacitance to the signals that pass through them. The average impedance resulting from the combination of such a socket, vias located on both the motherboard and a memory card, and the memory card""s edge fingers is a function of the socket""s inductance and the sum of the capacitance that these elements apply. (The impedance may be estimated by taking the square root of that inductance divided by that sum.)
When the inductance that the socket provides is relatively high, the average impedance over the motherboard to memory card connection may be higher than desired, as it may cause a signal trace, which passes from the motherboard to the memory card, to experience an unwanted impedance discontinuity. Accordingly, there is a need for an improved connection between a motherboard and a memory card. There is a need for such a connection that reduces impedance discontinuity over a memory channel that is capable of transmitting high speed, pipelined signals. There is a need for such a connection that may be easily integrated into a computer assembly that is made with standard tools and components. The present invention provides such a connection.